1. Field of the Invention
A can having a bottle-shape of aluminum alloy in which a cap is screwed on a mouth section having a screw is known as a container filled with contents such as drinks. The can is manufactured by: forming an aluminum alloy sheet into a closed-end cylindrical body which has a bottom plate and a cylindrical side surface in one piece by performing drawing processing and ironing processing (i.e., DI forming); coating an inner surface and an outer surface of the close-ended cylindrical body; forming a shoulder and the mouth section by performing so-called neck-in processing on an aperture section; and performing screw-forming processing, curl-forming processing and the like on the mouth section.
2. Description of Related Art
An inner coating in the can is formed by thermo-setting resin such as epoxy-acrylic resin, polyester resin or the like in order to make corrosion resistance and so on to the can with respect to content of the can (refer to Patent Document 1). The inner coating is formed by spraying paint on an inner surface of the can after the drawing and ironing processing before the neck-in processing. However, the paint may scatter around or may cleave to the outer surface of the can and tiny protrusions are formed, so that the protrusions may preface with corrugations when the neck-in processing is performed.
A curl portion is formed by folding an upper end of the mouth section outward and the inner coating is formed an outer surface of the curl portion. The can is hermetically sealed by attaching a cap so as to press a liner to the curl portion (refer to Patent Document 2). Therefore, if asperity such as the aforementioned corrugations by the paint is formed on the surface of the curl portion, especially on a top surface, the content may be leaked.
Therefore, it is important that the asperity such as the corrugations and the like are not formed on the top surface of the curl portion. Furthermore, in case of the asperity is formed, it is expected that the asperity is detected in an inspection process and reliably excluded as a defective.
In addition, there is a case in which asperity is formed on the surface of the curl portion by the inner coating being crumpled when the mouth section or the curl portion is formed.
For example, as a detection method for detecting fine asperity (e.g., corrugation or the like) formed on the outer surface, a detection method in which a can-body is irradiated obliquely to a tangent plane (i.e., a plane along a tangent line of the outer surface) so that corrugations are detected by observing reflection or shade of the corrugations along the tangent plane is suggested (refer to Patent Document 3).